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7 Sep 2006NVO Summer School 20064 What is a web service? A software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine- processable format. - W3C WSDL and SOAP conveyed using HTTP with an XML serialization + other Web-related standards Not a new idea: –RPC/RMI –CORBA –DCOM –XML-RPC

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7 Sep 2006NVO Summer School 20065 Web service models - I Resource oriented model –A resource is anything that can have an identifier (URI) –Focuses on resource description and representation (data object reflecting state of resource) Service oriented model –A service is realised by an agent (provider) and used by another (requester) –Focuses on tasks (unit of action) that may be performed by an agent: description, messaging/choreography and goal state

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7 Sep 2006NVO Summer School 20066 Web service models - II Message oriented model –A message is the basic unit of data sent from one agent to another: the action taken in response to receiving a message is up to the recipient –Focuses on message structure (contract) and transport (choice of protocol carries no semantics) Policy oriented model –A policy is a set of assertions expressing capabilities and constraints –Focuses on security, quality of service and management

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7 Sep 2006NVO Summer School 20067 Hows it done in the real world? Things should be made as simple as possible, but no simpler - Albert Einstein WWW is the largest, most distributed and scalable application on the planet: –Objects (resources) are identified by URIs –Resource state information (representations) is exchanged in many data formats –Protocols that support interaction between agents and resources - choice of protocol places limits on which representation formats can be transmitted HTTP and HTML (XML)

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7 Sep 2006NVO Summer School 200620 SOA and WOA An application architecture within which all functions are defined using a description language as independent services with invokable interfaces which can be called in defined sequences to form business processes. Principles: –Service reusability –Service contract –Service loose coupling –Service abstraction Virtual Observatory = Service Oriented Astronomy WOA advocates REST or POX over HTTP – Service composability – Service autonomy – Service statelessness – Service discoverability